Grain-Oriented Electrical Sheet Superior in Watt Loss

ABSTRACT

The present invention provides grain-oriented electrical sheet more superior in watt loss compared with the past by dividing the watt loss of grain-oriented electrical sheet introducing strain by firing of a laser beam etc. into hysteresis loss and eddy current loss and, in particular from the viewpoint of the eddy current loss, quantitatively suitably controlling the distribution of the strain and residual stress in the sheet thickness direction, that is, grain-oriented electrical sheet obtained by firing a laser beam etc. to introduce lines of strain substantially perpendicular to the rolling direction uniformly in a sheet width direction and cyclically in the rolling direction for magnetic domain control, characterized in that in the two-dimensional distribution of a rolling direction compressive residual stress occurring near one location of the introduction of strain in a cross-section perpendicular to the sheet width direction, the value of the rolling direction compressive residual stress integrated in the region of the cross-section where there is compressive residual stress is within a predetermined range.

TECHNICAL FIELD

The present invention relates to grain-oriented electrical sheet superior in watt loss which uses laser firing or the like to introduce residual stress for magnetic domain control.

BACKGROUND ART

Grain-oriented electrical sheet having an axis of easy magnetization in the rolling direction of the steel sheet is mainly being used for iron cores of transformers etc. In recent years, it has been strongly demanded to reduce the watt loss of iron cores from the viewpoint of energy savings.

The watt loss of electrical sheet may be roughly divided into hysteresis loss and eddy current loss. It is known that the hysteresis loss is influenced by the crystal orientation, defects, grain boundaries, etc., while the eddy current loss is influenced by the sheet thickness, electrical resistance, magnetic domain width, etc. There are limits to the technique of controlling and improving the crystal orientation so as to reduce the hysteresis loss, so in recent years many proposals have been made of the art of subdivision of the magnetic domain width so as to reduce the eddy current loss accounting for most of the watt loss, that is, the art of magnetic domain control.

As a method for this, Japanese Patent Publication (B2) No. 6-19112 discloses a method of production of grain-oriented electrical sheet which uses YAG laser firing to introduce lines of strain substantially perpendicular to the rolling direction cyclically in the rolling direction and thereby reduce the watt loss. The principle behind this method, called laser magnetic domain control, is to use a laser beam to scan the surface and produce surface strain due to which the 180° magnetic domain width is subdivided and the watt loss is reduced.

Further, Japanese Patent Publication (A) No. 2005-248291 makes a new proposal taking note of the maximum value of the rolling direction residual stress formed at the steel sheet surface.

DISCLOSURE OF THE INVENTION

Almost all proposals up to now relating to the introduction of local strain to steel sheet surfaces and subdivision of the 180° magnetic domain width to reduce the watt loss, that is, laser magnetic domain control, including the prior art first patent document, use trial and error to limit the type of the laser, the shape of the focused spot of the laser beam, the laser energy density, the laser firing pitch, and other laser firing parameters. The proposals are extremely fragmentary and lack uniformity. The reason is that no allusion is made to a quantitative discussion of the main factors causing magnetic domain subdivision and watt loss reduction, that is, strain or residual stress. Inherently, in improvement of watt loss by laser firing, even under the same laser firing conditions, due to the absorption rate of the steel sheet (determined by laser wavelength or surface properties, shape, and film composition) or film thickness, the conversion from laser energy to heat energy (temperature distribution and temperature history) will differ, so even if the laser firing conditions are the same, the strain introduced will differ depending on the properties of the steel sheet. Further, even with the same heat energy (temperature distribution or temperature history), due to the composition of the steel sheet (for example, amount of Si), the physical property values (for example, Young's modulus or yield stress value) will differ, so the residual stress will also differ. Therefore, even if the optimal laser firing conditions with respect to steel sheet of certain conditions are obtained, even a small change in the state of the film will cause the way the strain is introduced due to the laser to differ and the watt loss value to change, so the laser firing conditions and reduction in watt loss do not correspond to each other on a 1 to 1 basis. Therefore, attempts have been made to find the inherent factors influencing the watt loss. The second patent document quantitatively alludes to the strain and residual stress, but there were limits to reduction of the watt loss by just control of the strain or tensile residual stress of the steel sheet surface.

The object of the present invention is to provide grain-oriented electrical sheet more superior in watt loss compared with the past by dividing the watt loss of grain-oriented electrical sheet into hysteresis loss and eddy current loss and, in particular from the viewpoint of the eddy current loss, quantitatively controlling the distribution of the strain and residual stress not only at the surface, but also inside in the sheet thickness direction under suitable conditions.

The inventors ran experiments on magnetic domain control introducing strain and residual stress into grain-oriented electrical sheet by laser firing etc. and engaged in in-depth research to investigate the distribution of residual stress introduced into the obtained low watt loss grain-oriented electrical sheet. As a result, the inventors discovered a correlation between the residual stress and eddy current loss and discovered that if controlling the compressive stress value and the strain pitch, it is possible to realize a grain-oriented electrical sheet superior in watt loss. The gist of the present invention is as follows.

(1) A grain-oriented electrical sheet obtained by firing a continuous wave laser beam to introduce strain uniformly in a sheet width direction perpendicular to a rolling direction, cyclically in the rolling direction, and in lines substantially perpendicular to the rolling direction, characterized in that in the two-dimensional distribution of a rolling direction compressive residual stress occurring near one location of the introduction of strain in a cross-section perpendicular to the sheet width direction, the value of the rolling direction compressive residual stress integrated in the region of the cross-section where there is compressive residual stress is 0.20 N to 0.80 N.

(2) A grain-oriented electrical sheet as set forth in said (1), characterized in that a cyclic pitch in said rolling direction of the strain uniform in said sheet width direction due to firing of the laser beam is 2 mm to 8 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an apparatus used for a method of production of a grain-oriented electrical sheet of the present invention.

FIG. 2 shows a two-dimensional distribution of a rolling direction residual stress near a laser firing position at a rolling direction/sheet thickness direction cross-section.

FIG. 3 is a view of a relationship between a maximum value of a rolling direction tensile residual stress and a watt loss W_(17/50).

FIG. 4 is a view of a relationship between a cumulative compressive stress value σS and an eddy current loss We (laser firing pitch of fixed 4 mm).

FIG. 5 is a view of a relationship between a cumulative compressive stress value σS and a watt loss W_(17/50) (laser firing pitch of fixed 4 mm).

FIG. 6 is a view of a relationship between a laser firing pitch PL and a watt loss W_(17/50) (rolling direction laser firing diameter DL of 0.1 mm and scan direction laser firing diameter DC of fixed 0.5 mm).

FIG. 7 is a view of a relationship between a maximum value of a rolling direction compressive residual stress and a watt loss W_(17/50).

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors took note of the two-dimensional distribution of rolling direction residual stress in the cross-section vertical to the sheet width direction and the rolling direction laser firing pitch for various laser firing conditions in the method of firing a laser at the surface of grain-oriented electrical sheet so as to introduce lines of strain substantially vertical to the rolling direction at a constant pitch in the rolling direction so as to improve the watt loss and discovered conditions by which grain-oriented electrical sheet superior in watt loss can be obtained. Here, the “sheet width direction” is a direction perpendicular to the rolling direction. As the method for introducing lines of strain like the above at the surface of grain-oriented electrical sheet, in addition to the laser firing method, ion injection, electrodischarge machining, local plating, ultrasonic vibration, etc. may be mentioned. The conditions can be applied to grain-oriented electrical sheet introducing strain by any method. Below, drawings will be used to explain the grain-oriented electrical sheet obtained by laser firing of the present invention.

FIG. 1 is an explanatory view of the method of firing a laser beam according to the present invention. In the present embodiment, the continuous wave (CW) laser beam LB output from the laser device 3 is used to scan a grain-oriented electrical sheet 1 using a polygonal mirror 4 and fθ lens 5. By changing the distance between the fθ lens 5 and grain-oriented electrical sheet 1, the rolling direction focused diameter dl of the laser beam was changed. 6 is a cylindrical lens or a plurality of cylindrical combination lenses. This is used in accordance with need to change the focused diameter (scan direction length) dc of the scan direction of the beam (sheet width direction perpendicular to rolling direction) for the focused spot of the laser beam so as to control the focused shape from a circular shape to an elliptical shape. The average firing energy density Ua [mJ/mm²] is defined using the laser power P [W], sheet width direction scan speed Vc of the laser beam in the sheet width direction [m/s], and rolling direction laser firing pitch PL (mm) as

Ua(mJ/mm²)=P/(Vc×PL)

The laser scan speed is determined by the rotational speed of the polygonal mirror, so the average laser firing energy density can be adjusted by changing the laser power, polygonal mirror rotational speed, and laser firing pitch. FIG. 1 is an example of use of one set of a laser and laser beam scan device. It is also possible to set a plurality of similar devices in the sheet width direction in accordance with the width of the steel sheet.

The inventors ran experiments using a 10 μm fiber core diameter continuous wave fiber laser device, changed the laser firing conditions while combining the focused spot shape and average laser firing energy density Ua in various ways, and made the laser beam scan the surface of the grain-oriented electrical sheet in lines in a direction substantially vertical to the rolling direction so as to laser it. They measured the two-dimensional distribution of the residual stress in the rolling direction in the cross-section vertical to the sheet width direction and the watt loss and hysteresis loss and divided the watt loss into hysteresis loss and eddy current loss for study. For measurement of the two-dimensional distribution of the residual stress in the rolling direction in the cross-section vertical to the sheet width direction, they used the X-ray diffraction method to measure the lattice intervals and used the modulus of elasticity and other physical property values to convert this to stress. The watt loss was measured as W_(17/50) by an SST (Single Sheet Tester) measuring device. W_(17/50) is the watt loss at the time of a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T. In the grain-oriented electrical sheet sample used in this example, when the sheet thickness is 0.23 mm, the W_(17/50) before laser firing was 0.86 W/kg. The hysteresis loss was calculated by a hysteresis loop, while the eddy current was made the value of the watt loss minus the hysteresis loss.

FIG. 2 shows a typical example of the two-dimensional distribution of the compressive residual stress of the rolling direction occurring near the laser firing position in a cross-section vertical to the sheet width direction. For steel sheet where improvement in the watt loss is seen, there are differences in the absolute value of the residual stress depending on the laser firing conditions, but there is a large tensile stress near the surface of the steel sheet and there is compressive stress directly under the sheet thickness direction. Note that the width of the rolling direction in which the residual stress and plastic strain are present is substantially proportional to the rolling direction diameter dl of the focused spot of the laser.

The inventors investigated the relationship between the maximum value of the tensile residual stress and compressive residual stress of the surface of the steel sheet and the watt loss. The relationship between the maximum value of the tensile residual stress and the watt loss is shown in FIG. 3, while the relationship between the maximum value of the compressive residual stress and the watt loss is shown in FIG. 7. For the maximum value of the tensile residual stress, no correlation with the watt loss or optimal value is seen. On the other hand, for the maximum value of the compressive residual stress, the watt loss is good above the 100 MPa shown by the one-dot chain line, but the upper limit value is not clear. As a result, the watt loss in magnetic domain control by laser firing cannot be explained by the maximum value of the tensile residual stress and cannot be completely explained even by the maximum value of the compressive residual stress. The possibility of the presence of separate particularly fine amounts may be considered.

Therefore, the inventors studied the data in detail and as a result noted, as a first point, that the maximum value of the tensile residual stress is greater than the compressive residual stress and the tensile residual stress concentrates in a narrow region, that depending on the firing conditions, the yield stress, that is, plastic strain region, is reached, that, on the other hand, some relationship was seen between the maximum value of the compressive residual stress and the watt loss, and, as a second point, even if the maximum value of the compressive residual stress is the same, there is a difference in the spread of the distribution of compressive residual stress in the depth direction. That is, they began to believe that as the main factors behind the realization of reduction of watt loss and realization of magnetic domain subdivision are, from the first point, not the tensile stress, but the compressive stress has important meaning and, from the second point, not the maximum value of the residual stress, but the spread of the distribution has important meaning.

To express the distribution of compressive stress for realizing reduction of the watt loss, the inventors defined the characterizing quantity of the “cumulative compressive stress value σS” as in the following formula (1):

$\begin{matrix} {{\sigma \; S} = {\int_{S}^{\;}{\sigma \ {s}}}} & (1) \end{matrix}$

That is, in the two-dimensional distribution of the rolling direction compressive residual stress occurring near a lasered part, that is, near a part where strain is introduced, in the cross-section vertical to the sheet width direction, they defined the cumulative compressive stress value σS [N] as the value of the stress σ integrated in the region S where the rolling direction compressive residual stress is σ [MPa], the region in the cross-section in which there is compressive residual stress is S [mm²], and the area element_is ds. That is, the cumulative compressive stress value is the sum of the compressive residual stress introduced by laser firing.

The inventors found the cumulative compressive stress by the above method for grain-oriented electrical sheet obtained by setting the rolling direction laser firing pitch PL at 4 mm (fixed), setting the shape of the laser focused spot at 20×2500 μm, 100×500 μm, 100×2000 μm, and 300×200 μm, and changing the laser power for each in stages for the laser firing. On the other hand, they subtracted the hysteresis loss from the watt loss measured for each to find the eddy current loss. FIG. 4 shows the relationship between the two for each electrical sheet obtained by plotting the cumulative compressive stress value σS on the abscissa and the eddy current loss We on the ordinate. From the result, the cumulative compressive stress value and the eddy current loss are in an inversely proportional relationship regardless of the shape of the focused spot. This means that the reduction in the eddy current loss, that is, the magnetic domain subdivision effect, is proportional to the sum of the introduced compressive residual stresses. If considering this phenomenon from the physical principles, the result becomes as follows. The magnetic elasticity energy E is

E=−C×σ×M×cos² θ

where C is a constant, σ is the residual stress, M is the magnetic moment, and θ is the angle formed by σ and M. At this time, when there is compressive residual stress in the rolling direction, since E becomes smallest when θ is 90 degrees, σ is a negative value. If taking note of this, the orientation of the magnetic moment becomes vertical to the rolling direction. Therefore, due to the compressive stress, the axis of easy magnetization can be made not only the rolling direction, but also the vertical direction. In general, this is called a “reflux magnetic domain”. If there is a reflux magnetic domain, the magnetostatic energy becomes higher and unstable, so it may be considered to further divide the magnetic domains to lower the magnetostatic energy and stabilize it. Accordingly, it is believed, the greater the reflux magnetic domains, that is, the stronger and broader the compressive residual stress generated, the higher the magnetic domain subdivision effect becomes and the more the eddy current loss is reduced.

FIG. 5 shows the relationship when using the data used in FIG. 4 and the measured watt loss and plotting the cumulative compressive stress value σS on the abscissa and the peak watt loss W_(17/50) on the ordinate. From the results, in the range of 0.20 N≦σS≦0.80 N shown by the dot-chain line, compared with the watt loss W_(17/50)=0.86 W/kg before magnetic domain control, a good watt loss of a watt loss improvement rate of 13% or more (W_(17/50)≦0.75 W/kg) shown by the dotted line can be realized. Note that, the watt loss improvement rate η is defined as η(%)={(watt loss of material-peak watt loss)/watt loss of material}×100. If the cumulative compressive stress value σS is smaller than 0.20 N, the eddy current loss is high, so the watt loss is not reduced. It is believed that, when the cumulative compressive stress value σS is larger than 0.80 N, the eddy current loss is reduced, but the hysteresis loss increases due to the plastic strain due to the tensile residual stress near the surface, so the watt loss is not reduced. In the above way, it is learned that if adjusting the cumulative compressive stress value σS to the range of

0.20 N≦σS≦0.80 N

a good improvement in the watt loss is obtained. More preferably, it is learned that if adjusting the value to the range of 0.40 N≦σS≦0.70 N, a further effect of improvement of the watt loss can be obtained.

In the above, the rolling direction laser firing pitch PL was fixed at 4 mm, but the inventors further investigated the effects by changing the rolling direction laser firing pitch PL. At this time, they made the shape of the focused spot of the laser beam a rolling direction diameter of 0.1 mm and a scan direction (sheet width direction) diameter of 0.5 mm and adjusted Ua so that the cumulative compressive stress σS fell in the range of 0.20 N≦σS≦0.80 N. FIG. 6 plots the rolling direction laser firing pitch PL on the abscissa and the watt loss W_(17/50) on the ordinate and shows the relationship between the two. From the results, with a PL of 2 mm to 8 mm, a good watt loss of a watt loss improvement rate of 13% can be realized. In a range where PL is smaller than 2 mm, the hysteresis loss increases, so the watt loss is not reduced. In a range where PL is larger than 8 mm, the eddy current loss is not reduced, so the watt loss is not reduced. In the above way, it is learned that if adjusting the rolling direction laser firing pitch PL to the range of

2 mm≦PL≦8 mm

a good improvement in the watt loss can be obtained.

Example 1

Using 0.23 mm thick grain-oriented electrical sheet, the surface of the steel sheet was scanned using a continuous wave laser under the laser firing conditions as shown in Table 1, the residual stress was measured, then the cumulative compressive stress value was calculated and the watt loss (W_(17/50)) was measured. The results are shown in together in the same Table 1. Example 1 was performed fixing the laser power at 200 W and the laser firing pitch in the rolling direction at 4 mm. The cumulative compressive stress value was calculated by using the X-ray diffraction method to measure the rolling direction residual stress (strain) and finding the value with respect to the compressive stress by formula (2).

As clear from Table 1, the electrical sheets shown in Test No. 1 to No. 8 (invention examples) all had a rolling direction cumulative compressive stress value σS in the range prescribed by the present invention, that is, 0.20 N≦σS≦0.80 N, so could be reduced in watt loss to a low watt loss value (W_(17/50)) of 0.75 W/kg, for a watt loss improvement rate of 13%, or less. On the other hand, the electrical sheets shown in Test No. 9 to No. 12 (comparative examples) outside the range of conditions 0.20 N≦σS≦0.80 N failed to achieve a low watt loss value (W_(17/50)) of 0.75 W/kg or less. In this way, if using the present invention, it is possible to obtain grain-oriented electrical sheet superior in watt loss.

TABLE 1 Rolling Scan Cumulative direction direction Average Strain Maximum compressive Watt loss Watt loss diameter diameter energy pitch tensile stress value improvement Test DL DC density Ua PL stress value σS W17/50 rate No. mm mm mJ/mm² mm MPa N W/kg % Not lasered 0 — — — — 0 0 0.860 0 Inv. ex. 1 0.020 2.50 2.5 4 370 0.30 0.730 15.1 Inv. ex. 2 0.020 2.50 3.5 4 350 0.50 0.716 16.7 Inv. ex. 3 0.100 0.50 1 4 460 0.45 0.725 15.7 Inv. ex. 4 0.100 0.50 2 4 450 0.55 0.715 16.9 Inv. ex. 5 0.100 2.00 2 4 400 0.38 0.730 15.1 Inv. ex. 6 0.100 2.00 2.5 4 400 0.45 0.710 17.4 Inv. ex. 7 0.300 0.20 2 4 420 0.58 0.730 15.1 Inv. ex. 8 0.300 0.20 3 4 410 0.70 0.735 14.5 Comp. ex. 9 0.020 2.50 1 4 330 0.10 0.820 4.7 Comp. ex. 10 0.100 0.50 4 4 440 0.85 0.755 12.2 Comp. ex. 11 0.100 2.00 1 4 390 0.14 0.800 7.0 Comp. ex. 12 0.300 0.20 4 4 410 0.90 0.765 11.0

Example 2

The surface of 0.23 mm thick grain-oriented electrical sheet was scanned by a continuous wave laser beam under the laser firing conditions as shown in Table 2, the residual stress of the lasered part was measured, then the cumulative compressive stress value was calculated and the watt loss (W_(17/50)) was measured. These values are shown in together in Table 2. Example 2 was performed fixing the laser power at 200 W the same as Example 1.

As clear from Table 2, the electrical sheets shown in Test No. 1 to No. 6 (invention examples) all have a rolling direction cumulative compressive stress value σS and a rolling direction laser firing pitch (strain pitch) PL in the ranges prescribed in the present invention, that is, 0.20 N≦σS≦0.80 N and 2 mm≦PL≦8 mm, so could be reduced in watt loss to a low watt loss value (W_(17/50)) of 0.75 W/kg, for a watt loss improvement rate of 13%, or less. On the other hand, the electrical sheet shown in Test No. 7 and No. 8 having a cumulative compressive stress value σS satisfying the conditions, but off from the conditions of the firing pitch PL failed to achieve a low watt loss value (W_(17/50)) 0.75 W/kg or less. In this way, if using the present invention, it is possible to obtain grain-oriented electrical sheet superior in watt loss.

TABLE 2 Rolling Scan Cumulative direction direction Average Strain Maximum compressive Watt loss Watt loss diameter diameter energy pitch tensile stress value improvement Test DL DC density Ua PL stress value σS W17/50 rate No. mm mm mJ/mm² mm MPa N W/kg % Not lasered 0 — — — — 0 0 0.860 0 Inv. ex. 1 0.100 0.20 1.5 2 340 0.45 0.735 14.5 Inv. ex. 2 0.100 0.50 1.5 2 450 0.22 0.740 14.0 Inv. ex. 3 0.100 0.50 1.5 4 440 0.50 0.720 16.3 Inv. ex. 4 0.100 0.50 1.5 6 460 0.65 0.730 15.1 Inv. ex. 5 0.100 0.50 1.5 8 450 0.75 0.745 13.4 Inv. ex. 6 0.100 2.00 3 8 390 0.23 0.748 13.0 Inv. ex. 7 0.100 0.50 1.5 1 330 0.21 0.755 12.2 Inv. ex. 8 0.100 0.50 1.5 10 430 0.80 0.760 11.6

INDUSTRIAL APPLICABILITY

According to the present invention, by quantitatively suitably controlling the residual stress introduced to the grain-oriented electrical sheet, in particular the compressive residual stress, it is possible to obtain to stably obtain grain-oriented electrical sheet superior in watt loss compared with the past. If using the grain-oriented electrical sheet of the present invention as an iron core, a high efficiency, small-sized transformer can be produced. The value of industrial application of the present invention is extremely high. 

1. A grain-oriented electrical sheet obtained by firing a continuous wave laser beam to introduce strain uniformly in a sheet width direction perpendicular to a rolling direction, cyclically in the rolling direction, and in lines substantially perpendicular to the rolling direction, characterized in that in the two-dimensional distribution of a rolling direction compressive residual stress occurring near one location of the introduction of strain in a cross-section perpendicular to the sheet width direction, the value of the rolling direction compressive residual stress integrated in the region of the cross-section where there is compressive residual stress is 0.20 N to 0.80 N.
 2. A grain-oriented electrical sheet as set forth in claim 1, characterized in that a cyclic pitch in said rolling direction of the strain uniform in said sheet width direction due to firing of the laser beam is 2 mm to 8 mm. 